In many facilities there is a need for providing heated water, e.g. in homes, work places, factories. In large facilities it is often an advantage to use a heat pump, which produces both cold and hot water for HVAC systems in e.g. hotels. It is meant to replace the boiler in the building where it is installed, or at least save fossil fuels.
A particular problem is that the demand for hot water in for example a hotel varies greatly during the day. See for example FIG. 1 of the present application where a typically hot water consumption profile of a large facility is shown during 24 hours.
In order to meet the hot water demand, heat pump systems with three interconnected heat exchangers are known. They are called ‘tri-lobe’ heat pumps designed to optimize the coefficient of performance for the heat pump system. Such a tri-lobe heat pump system is for example shown in German utility model DE 20 2005 013 499, where a refrigeration cycle for a heat pump comprising an evaporator and an expansion valve, a compressor, and three capacitors or condensers are arranged therein and forming part of the refrigerant circuit or line.
Despite carefully considerations when designing a tri-lobe based heat pump system, it is nevertheless difficult in practise to fulfil the demand represented by a typically irregular demand cycle, cf. FIG. 1, and at the same time maintain a high coefficient of performance (COP) for the overall heat pump system in order to save energy.
International patent application WO 93/07424 (to Egmont Bartl) discloses a plant for recuperating heat from waste water. The waste water flows through a heat exchanger which is also connected to a hot water tank. The temperature differences between the inlet of waste water and outlet of heated water, and the outlet of waste water and the inlet of water to be heated, respectively, are measured and applied for optimizing heat exchange by varying corresponding supply pumps on both the primary and the secondary side of the heat exchanger so as to keep the temperature differences at an equal level. However, simulations performed by the present inventor shows that the applied control algorithm of WO 93/07424 results in a sub-optimum heat exchange, and a corresponding low COP.
Hence, an improved method for controlling a pump for feeding fluid, e.g. city water, into a heating system would be advantageous, and in particular a more efficient and/or reliable method would be advantageous.